1. Field of the Invention
The present invention relates to the analysis of substances using electromagnetic radiation, and more particularly to a sample holder for X-ray diffractometry.
2. State of the Art
X-ray diffractometry, using the phenomenon of X-ray diffraction, is a well-known technique for performing physical and chemical analysis of crystalline substances. In X-ray diffraction, the crystal lattice of the crystalline substance acts in relation to X-rays in an equivalent manner as a diffraction grating acts in relation to visible light. Each unit cell of which a crystalline structure is composed acts as a diffraction center. The radiation intensity at a point removed from the crystalline structure is determined by the phase difference and intensity of radiation diffracted from each diffraction center to the point in question. A diffracted X-ray beam is essentially a collection of a large number of scattered X-ray waves that satisfy the conditions of constructive interference. These conditions are specified by Bragg's law, which may be stated as 2d=.lambda. sin .theta. where d is the interplanar spacing of unit cells, .lambda. is the X-ray wavelength, and .theta. is the angle between the X-ray waves and a diffraction plane.
X-ray diffraction analysis may be performed on either single crystal substances or on crystalline powders. In a typical X-ray diffraction analysis, a powder sample is placed in a sample holder. The sample holder may be "front loading" or "back loading". A front loading sample holder is usually simply a plate with a depression at the center. The depression is filled with sample and excess sample is removed by scraping. A back loading sample holder is usually a plate with a large hole drilled through at the center. The hole is filled completely with sample powder from the backside, and a backplate is placed on the backside. The holder is then up-righted, and the front plate is removed to expose the sample. It is generally recognized that the back loading sample holder is effective in preventing preferred particle orientation, which is undersiderable, since X-ray diffractometry assumes that the crystals of a powder sample are randomly oriented.
During analysis, as shown in FIG. 1, the sample is irradiated with an X-ray beam 11 produced by an X-ray source 13 and incident at an angle. The X-ray 14 diffracted by the sample 15 held in the sample holder 17 is detected by a detector 19 positioned at a diffracted angle equal to the incident angle. The incident angle and hence the diffracted angle are incrementally varied as both the X-ray source 13 and the detector 19 move (in the plane of the figure) along a semicircle above the sample. The movements are synchronized so that the incident angle and the diffraction angle are always equal. The X-ray diffractometer records a diffractogram, which is the intensity of the diffracted X-ray as a function of angle. The sample can be identified based on its diffractogram.
A conventional front loading powder holder 21 is shown in FIG. 2. Commercially available sample holders are usually made of glass. There are also sample holders made of single crystal quartz in order to minimize background noise.
There are several disadvantages of most commercially available sample holders. First, the sample is exposed to air. A typical X-ray diffraction analysis may require from a few minutes to a few hours. To improve the signal-to-noise ratio of the detected signal, a large number of scans may be performed and signals detected at each scan added together. Air or moisture sensitive samples will be destroyed (chemically altered) during the analysis. One obvious way to protect the sample is to cover the sample with a metal or plastic film thin enough to allow the X-ray beam to pass through. The problem with this is that the film is also in the sampling point of the diffractometer. That is, X-rays diffracted by the sample cover will be detected by the X-ray detector. As a result, the background noise from the cover film cannot be distinguished from the signal of the sample.
Second, most commercially available sample holders are fixed-volume. Different sample holders must be used for samples of different size to achieve the best signal-to-noise ratio. To handle small samples, a sample holder with a small depression is necessary. But with a small depression, the area of the holder surrounding the sample, as well as the bottom of the depression, are also exposed to the X-ray beam. Unless the sample holder is made of specially prepared single crystal quartz, it will contribute to the background noise.
Accordingly, what is needed is an improved sample holder for diffraction analysis that protects air or moisture sensitive samples without degrading the detected signal. Furthermore, a sample holder is needed that accommodates samples of different sizes without contributing to background noise.